An oil pump is generally operatively connected to the output shaft of the prime mover of a chain saw in order to feed lubricating oil between the saw chain and the guide bar on which it is held. The oil pump case is secured to a side of the prime mover by inserting fixing screws through holes provided in the case body and screwing the fixing screws into threaded holes provided on the side of the prime mover. A dust preventing and thrust receiving plate, covering the area of the output shaft at the tip of the case body, is press fit onto the output shaft.
Therefore, in order to remove the oil pump for maintenance or the like, the case body and the cover plate must first be removed from the output shaft. However, as the cover plate is press fit onto the output shaft, it will be very difficult to remove them, unless special measures are taken.
Therefore, in the conventional chain saw, extra screw holes receiving disassembling push screws have been provided in addition to the inserting holes in the case body of the oil pump. Thus when the case body and the cover plate are to be removed, the disassembling push screws are inserted into the extra disassembling holes so that the case may be pushed up and away from the prime mover. That is to say, after the fixing screws are removed, and the disassembling push screws are inserted into the extra disassembling screwed holes, the push screws will move relatively with the case body. The tips of the push screws contact and bear against the side wall of the prime mover so that upon further screwing, the case will move outwardly toward the tip end of the output shaft and the case body and the cover plate will be able to be removed.
However, in the above mentioned conventional chain saw, as the extra disassembling holes are provided in addition to the fixing holes the number of manufacturing steps and the cost are increased.
Also, a disadvantage arises in that saw dust or the like will accumulate in the extra holes.
Incidentally, in the above mentioned conventional chain saw, the fixing screw inserting holes provided in the case body of the oil pump are not threaded holes but are through holes.